1. Field of the Invention
The invention relates to a method for feeding a fibrous suspension and, in particular, to feeding a fibrous suspension to a headbox of a paper or cardboard machine or to feeding a fibrous suspension to at least one filter device and degassing device. The invention also relates to a degassing device for carrying out the above noted methods.
2. Discussion of Background Information
Methods of this type are used, for example, to supply paper or cardboard machines with fibrous suspension. The headbox delivers a fibrous layer onto a passing screen that has a thickening function. The stock feeding systems required thereto are known in principle. The fibrous suspension, which is to be fed, receives a major part of the fibers from a high-consistency suspension provided in a stock preparation unit. The high-consistency suspension has, e.g., a typical consistency between 2.5 and 5%. However, with the addition of a diluting liquid, such as a paper machine's backwater, the consistency of the high-consistency suspension is lowered to a value that is favorable for operating the headbox of the paper machine. While the backwater is optimally suited for this diluting task, there are problems due to the high gas content when the predominant part is air. The largest proportion of these gases escape very quickly, however, the residual gases often have to be removed in a complex manner so that the quality of the paper produced is not unduly lowered. Known solutions are large degassing containers in which negative pressure is permanently maintained by evacuation, wherein the negative pressure corresponds to the steam pressure of the suspension to be degassed. This is effective, but expensive.
A degassing pump for paper production is known from U.S. Pat. No. 6,723,205 B1, wherein the pump is provided with a rotor forming a cylindrical interior, in which the fed liquid can be degassed by centrifugal force. The backwater fed from the paper machine at high speeds provides the drive power for the rotor via a turbine, which is integrated into the degassing pump. When the backwater has left the degassing pump, the liquid is guided directly to a mixing station where raw fiber material (high-consistency stock) is added.
Another application for methods of this type occurs in the preparation unit (“stock preparation”), where the fibrous suspension is designated for processing on the paper or cardboard machine. Filter devices, and in particular disk filters, are used to increase the consistency of suspensions with relatively low consistency, e.g., between 0.5% and 1.5%. The fibrous suspension to be thickened can thereby have a high percentage of air content. For example, a high air content may exist if the fibrous suspension had previously been floated to eliminate contaminants with the flotation foam. The operation of disk filters and pumps is adversely affected by higher air contents.